Nine killed in clashes between army, Islamist group in Benghazi, Libya

Government is engaging fighters from Ansar al-Sharia, which the U.S. says attacked its consulate

"This is really the heaviest fighting" since the revolution, a resident says

A Libyan military official says forces are responding after an attack on army members

Tripoli, Libya (CNN) -- Heavy fighting continued Monday in Benghazi between Libyan army forces and an Islamist militant group the United States blames for the attack last year on its diplomatic mission that killed four Americans, including Ambassador Christopher Stevens.

LANA, Libya's state news agency, reported that at least six soldiers and one civilian were killed and 39 people were injured, most of them soldiers, in the clashes Sunday between the military and the group Ansar al-Sharia. On Monday, the Libyan government said that at least nine people were killed and 49 injured.

A senior Libyan military official in the city, speaking to CNN on condition of anonymity for security reasons, said heavy fighting continued Monday morning as troops engaged members of the group in different parts of Benghazi.

At midday, residents and state media reported a tense calm in the city.

Schools in Benghazi were closed, and security forces, through alerts on state media, asked residents to remain indoors Monday morning as troops worked to secure the city.

The fighting was not a planned operation but a "reaction" to an attack on the army by members of the group Sunday night, the official said. He said members of a military special forces group called Thunderbolt, with support from local residents, responded with raids on Ansar al-Sharia locations in Benghazi.

The official said the fighting Monday morning was intense, with Ansar al-Sharia fighters using mortars, rocket-propelled grenades and heavy weapons. He described the situation as very tense after reports that Ansar al-Sharia might be receiving reinforcements from the eastern city of Derna, where jihadist groups are active.

Libyan state media reported that military forces were securing all main roads and entrances to the city.

A Benghazi resident told CNN that heavy clashes broke out Sunday night, and gunfire and explosions intensified Monday morning across the city.

"This is really the heaviest fighting I have heard in Benghazi since the revolution" he said "Ansar al-Sharia versus the army, this has been a long time coming."

Militant group returns

After the assault on the U.S. Consulate last year, Benghazi residents attacked the headquarters of Islamist militias in the city, including Ansar al-Sharia's base. The group left the base but later returned.

During a visit to Benghazi this year, a CNN team confirmed that Ansar al-Sharia was back at its base and the group had a checkpoint at a western entrance to the city.

The government has been struggling to control the hundreds of militia groups that operate freely across the country, including some radical militant groups with ties to al Qaeda in eastern Libya.

Over the past year, there has been a significant deterioration in the security situation in Benghazi, which is Libya's second-largest city and the cradle of the 2011 revolution.

After an uptick in violence in the city, most notably in an assassination campaign that has primarily targeted security force members, most of whom held positions under the former regime, the government announced a security plan to secure the city.

This month, Benghazi residents reported an increased Libyan army presence, most notably the Thunderbolt forces working to secure the city.

Bombings and shootings have targeted army checkpoints and patrols, and a senior military official escaped an assassination attempt when a bomb targeted his car in Benghazi this month.